Corona discharge device

ABSTRACT

To provide a substantially uniform electrostatic charge on a relatively large area of a surface at ambient pressure, a novel corona discharge device comprises a transparent electrically conductive plate and a pointed member of electrically conductive material extending perpendicularly from the center of the plate and spaced from the surface. A source of high voltage is connected between the plate and the surface.

1 51 May 7,1974

United States Patent 1191 Gange et al.

[ 1 CORONA DISCHARGE DEVICE 5/1958 Walkup.....;..................... 317/262 A Inventors: Robert Allen g Ben-e Mead; 5/1960 Walkup....................... 317/262 A gtliralrlles Steinmetz, Mercerville, both Primary Examiner l D Miller Assistant Examiner-Harry E. Moose, Jr.

[73] Assignee: RCA Corporation, Princeton, NJ. Attorney, Agent, or Firm-H. Christoffersen; R. P.

Feb. 26, 1973. Williams; A. I. Spechler Appl. No.: 335,524

[22] Filed:

ABSTRACT To provide a substantially uniform electrostatic charge on a relatively large area of a surface at ambient pressure,-a novel corona discharge device comprises a Field of Search......... 317/262 A; 96/1 C, 27 H; transparent electrically conductive plate and a pointed 6 2. 7. 1 m G QML Um 2 M 55 [.1

member of electrically conductive material extending perpendicularly from the center of the plate and References Cited spaced from the surface. A source of high voltage is UNITED STATES PATENTS connected between the plate and the surface.

3,711,710 250/326 1 Claim, 1 Drawing Figure CORONA DISCHARGE DEVICE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, public law 85-568 (72 STAT 435; 42 U.S.C. 2457).

This invention relates generally to corona discharge devices, and more particularly to a corona discharge device adapted to provide a substantially uniform electrostatic charge on a surface at ambient pressure. The novel corona discharge device is particularly useful in methods of forming phase modulating holograms on deformable thermoplastic materials.

It has been proposed to form ripple images in a plurality of small regions in a thermoplastic layer of a recording device by providing electrostatic charge patterns of the images on the thermoplastic layer, and selectively heating the recording device, as with a laser beam, to form the ripple images in the softened thermoplastic layer at the desired locations. When the selected locations of the thermoplastic layer have cooled, the ripple images remain frozen therein. Unless, however, the electrostatic charge provided on the thermoplastic layer is uniform, the resulting developed ripple images will not be true replicas of the images it is desired to reproduce. Prior-art corona discharge devices, using two dimensional grids constructed over areas in excess of four square centimeters and employing various wires ranging in size from 1.8 to 5.0 mils (0.0046 to 0.0127 cm) in diameter, do not provide a uniform electrostatic charge distribution over four square centimeters. Although some improvement is obtained by using grids of finer mesh, the constraint of random access over a four square centimeter surface area limits the number, diameter, and substrate spacing of the wires that can be employed in practical corona discharge devices.

Briefly stated, the novel corona discharge device comprises an electrically conductive plate, a pointed member of electrically conductive material extending transversely from the plate and being electrically connected thereto, and means to apply a source of corona producing voltage to the plate.

The single FIGURE of the drawing is a vertical crosssectional view through the center of a substantially symmetrical novel corona discharge device disposed in an optical system with provisions for writing and erasing phase-type holograms.

Referring now to the drawing, there is shown a preferred embodiment of the novel corona discharge device 10. The device is substantially symmetrical on both sides of its vertical cross-section. The device 10 comprises a front wall 12 of a transparent plastic material, such as "Plexiglass, that is a good electrical insulator. The front wall 12 is, for example, about 10 centimeters square and is formed with a central square opening 14 of about 6 centimeters square.'The front wall 12 has an outer surface 16 and an inner surface 18. A square recording device 20 is disposed in a recess 22 in the inner surface 18 adjacent the opening 14 of the front wall 12.

The square recording device 20 typically comprises a substrate 23, a transparent coating 24 of electrically conductive material, a photoconductive layer 26, and a layer 28 of thermoplastic material. The substrate 23 is preferably transparent and may comprise glass or a transparent plastic material, such as Mylar, for example. The conductive coating 24 ispreferably a coating of InO, having a conductivity of about 100 ohms per square. The conductive coating 24 may also be .a thin transparent coating of a vacuum deposited metal, such as aluminum, for example. The photoconductive layer 26 is preferably a layer of polyvinyl carbazole (PVK), having a thickness of about 2 microns. The thermoplastic layer 28 can be a microcrystalline straight chain wax, having a thickness of between 1 to 1 A microns.

The recording device 20 is held in the recess 22 by any suitable means as, for example, by screws and washers adjacent each of the comers of the recording device 20. Thus, for example, upper and lower screws 30 and 32 and washers 34 and 36 hold the recording device 20 in the recess 22. Two other screws (not shown) are disposed adjacent the other two corners of the square recording device 20 in the other half (not shown) of the symmetrical device 10.

Means are provided to apply a voltage to the recording device 20 and also to send current through the conductive coating 24 to heat the recording device 20 so as to cause the thermoplastic coating 28 to soften for the purpose of making a ripple image therein. To this end, an electrical conductor 38 is connected to the lower screw 32 by means of a nut 40. A conductor 42 is also connected to the upper screw 30 by means of a nut 44. A pair of chromium-gold lands 46 and 48 are disposed on the conductive coating 24, adjacent the upper and lower edges of the photoconductive layer 26. The upper and lower washers 34 and 36 are adapted to contact the upper and lower lands 46 and 48 so that the conductor 38 can be connected to the conductor 42 through the conductive coating 24 and the screws 32 and 30.

Means are provided to apply a substantially uniform electrostatic charge on the surface 50 of the thermoplastic layer 28 of the recording device 20. To this end, a rear wall 52, substantially similar to the front wall 12, is parallel to, and adjustably spaced from, the front wall 12. A separate screw is threadd adjacent each of the corners of the front wall 12 and each of the screws extends outwardly from the inner surface 18. Thus, in the symmetrical half of the device 10 shown, an upper screw 54 and a lower screw 56 (two of four such screws of the device 10) extend outwardly from the inner surface 18 of the front wall 12. The rear wall 52 is formed with four holes adjacent each of its corners and adapted to receive the screws slidably therein. Thus, the upper screw 54 passes through a hole 58, and the lower screw 56 passes through a hole 60 in the rear wall 52. The rear wall 52 is urged away from the front wall 12 by springs around the screws, such as a spring 62 around the upper screw 54 and a spring 64 around the lower screw 56. The rear wall 52 can be moved toward and away from the front wall 12 by turning wing nuts 66 and 68 threaded on the screws 54 and 56.

The square rear wall 52 is formed with a square opening 70 that is substantially aligned with the opening 14 in the front wall 12. The rear wall 52 has an outer surface 72 and an inner surface 74. A square conductive plate 76 is supported in a recess 78 formed in the inner surface 74 of the rear wall 52 adjacent the opening 70. The conductive plate 76 comprises a sheet 80 of transparent material, such as glass, having a conductive coating 82, such as tin oxide or indium oxidethereon.

A pointed member 84, such as an electrically conductive needle point, about mm in length, is disposed in the center of the transparent conductive plate 76 through a hole formed, e.g. ultrasonically drilled, in the sheet 80; The pointed member 84 extends perpendicularly'from the conductive plate 76, points toward the recording device 20, and is electrically connected to the electrically conductive coating'82 by means of 78. A separate screw and washer, such as the screw 94 and washer 96, is disposed at each of the other corners of the conductive plate 76 to hold it firmly within the recess 78 of the rear'wall 52.

The novel corona discharge device 10 can be used in a method of forming a phase modulating hologram on the deformable thermosplastic layer 28of the recording device 20, as described, for example, in US. Pat. No. 3,560,205, issued to J. C. Urbach, on Feb. 2, 1971, for METHOD OF FORMING A PHASE MODULAT- ING HOLOGRAM ON A DEFORMABLE THERMO- PLASTIC, which method is incorporated herein by reference.

In operation, the rear wall 52 is spaced from, and disposed parallel to, the front wall 12 by adjusting the wing nuts 66 and 68 (and the wing nuts, not'shown) until the pointed member 84 is adesired distance from the surface 50 of the thermoplastic layer 28 to be charged. In practice, it has been found that when the pointed member 84 is about three centimeters from the 'surface 50, and a corona producing, unidirectional voltage of about 15 kilovolts is applied between the conductors 86 and 38, a four square centimeter area on the surface 50 will be substantially uniformly charged. The pointed member 84 is usually positive with respect to the conductive coating 24 (when the photoconductive layer 26 is PVK) but the polarity may be reversed if so desired. The further the pointed member 84 is from the surface 50 of the recording device 20, the greater the voltage which must be applied between the conductors 86 and 38.

' When the desired area on the surfaces 50 of the thermoplastic layer 28 is uniformly charged, it is exposed with an information light beam 94 which passes through a lens system 96 (illustrated here as a single lens) and through the transparent plate 76. The surface 50 is also exposed simultaneously with a reference light beam 98 which passes through a lens system 100 (illustrated here as a single lens) and through the transparent conductive plate 76 to form an interference pattern with the information beam 94 at the surface 50 the thermoplastic layer 28. The interference pattern discharges the electrostatically charged surface 50 selectively by rendering conductive theilluminated portions of the photoconductive layer 26. A source of voltage (not shown) applied between the conductors 38 and 42 causes the thermoplastic layer 28 to heat and to deform in accordance with the interference pattern on the surface 50, thereby producing a hologramv thereon.

The novel corona discharge device 10 can also be used with a lens system 102 (not shown herein as'a single lens). Thus, the surface 50 of the recording device 20 can be uniformly charged in the manner heretofore described and then exposed by an image through the lens system 100. Since the recording device 20 is substantially light transmitting, the image of an object 104 can be used to expose the photoconductive layer 26 of the recording device 20" after the thermoplastic layer 28 has been uniformly charged. Thus, a ripple image of the object 104 can then be formed in the thermoplastic layer 28 by heating the recording device 20 to deform it selectively. When cooled, the ripple image is frozen in the thermoplastic layer 28.

The transparent conductive plate 76 serves at leastthree functions. Firstly, the plate 76 serves as a transparent window through which images can be projected onto the surface 50. The pointed conductive member 84 is relatively very small compared to the relatively large surface area of the plate 76, and, therefore, is out of focus on the surface 50 when a light image is proje cted through the plate 76 and focused upon the surface 50. Secondly, the conductive plate 76 is parallel to the surface 50 and thereby functions to distribute the electric field emanating from the pointed member 84 substantially uniformly over the surface 50. The plate 76 also provides a uniformly charged background so that the corona discharge from the conductive member 84 is directed from the point of the conductive member 84, in a manner whereby a substantially uniform distribution of electrostatic chargev appears on the surface 50. Thirdly, the conductive plate 76 serves as a substantially two-dimensional (barrier) conductor which electrostatically isolates the space of corona discharge from the conductor 86, the high voltage power supply conductor. In the absence of the conductive plate 76, the electric field from conductor 86 could enter the corona discharge space in an asymmetric manner, thereby distorting the corona discharge pattern and producing a non-uniform distribution of electrostatic charge on the surface 50.

What is claimed is:

1. A corona discharge device for applying an electrostatic charge to a surface, said device comprising:

an electrically conductive plate, said plate comprising a sheet of transparent insulating material having a coating of transparent electrically conductive material thereon,

a pointed member of electrically conductive material extending transversely from said plate and being electrically connected to said transparent electrically conductive material, said pointed member being disposed in the center of said plate and extending perpendicularly thereto with its point pointed toward said surface,

means including said coating of transparent electrically conductive material, to apply a source of volt age between said pointed member and said surface, and

said device having adjustable means to position said plate substantially parallel to, and selectively spaced from, said surface. 

1. A corona discharge device for applying an electrostatic charge to a surface, said device comprising: an electrically conductive plate, said plate comprising a sheet of transparent insulating material having a coating of transparent electrically conductive material thereon, a pointed member of electrically conductive material extending transversely from said plate and being electrically connected to said transparent electrically conductive material, said pointed member being disposed in the center of said plate and extending perpendicularly thereto with its point pointed toward said surface, means including said coating of transparent electrically conductive material, to apply a source of voltage between said pointed member and said surface, and said device having adjustable means to position said plate substantially parallel to, and selectively spaced from, said surface. 